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What does "effective translational lift" (ETL) mean for a rotorcraft or multirotor transitioning to forward flight?

AETL is the maximum endurance speed: the airspeed at which hovering efficiency converts to forward-flight efficiency.

BETL occurs at approximately 16–24 knots forward airspeed when the rotor system moves into undisturbed air, creating an efficiency gain and allowing reduced power for the same lift.✓

CETL is the altitude at which translational thrust overcomes induced drag.

Why →ETL (Effective Translational Lift) is experienced as forward airspeed increases to approximately 16–24 knots. At that point, the rotor moves through undisturbed air rather than its own recirculating downwash. Rotor efficiency improves noticeably: the same thrust requires less power, or the same power generates more lift. For multirotors, this manifests as reduced motor current draw and improved endurance during slow forward flight vs. hover.

The trap →Maximum endurance speed is a different concept, and ETL has nothing to do with altitude. The marker is the 16–24 knot threshold: below it, the rotor is still fighting its own dirty air.

Field note →ETL is why consumer drones show measurably better endurance at 10–20 mph forward than in a static hover. If the mission allows slow forward flight (inspection passes, orbits, transit), take the battery savings.

SOURCE → FAA Helicopter Flying HandbookCHECKED JUL 16ACS IV.A.K1MED

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